History and Description of Downeast Boat Hull Design

Downeast Yachts & Lobster Boats For Sale

History of Lobster Boat Design – The Downeast lobster boat really came into being with the introduction of the Make and Break engines and the transition from sail and oar to power at the turn of the 20th Century. Maine boat builders took note of the early wooden power boats incorporating the higher powered motor car gasoline engines as seen in Motor Boat Magazine and other periodicals at the end of WWI. Will Frost of Jonesport, Maine in the 1920’s with his designs for working motor boats may be more responsible than any other designer for the present day lobster boat. The Downeast design came to incorporate a fine bow entry, generous deadrise amidships, and then a flattening out toward the stern, having a full length keel and large amount of cockpit or deck space. Today, the single engine Downeast Lobster boat is recognized as an efficient and able design.Modern Downeast boats are typically designed with hulls that are semi-displacement, or “semi-planing.” This hull type is capable of developing a moderate amount of hydrodynamic lift due to it’s underwater shape, however, most of the boat’s weight is still supported through buoyancy with typical power applications. When at rest, the vessel’s weight is supported by buoyancy equal to the vessel’s weight. At low speeds, most hulls act as displacement hulls where buoyancy is the predominant supporting force. As speed increases, there is an increase in hydrodynamic lift, buoyant support will decrease as the hull is lifted out of the water. At the same time the volume of water that the vessel displaced is decreased. When lift is the predominant upward force, the hull is considered to be planing. Downeast type hulls make use of this planing or semi-planing effect with the shape of their hulls where a flatter section with low dead rise is incorporated in the after section of the hull providing a means to transition from displacement speed to semi-displacement or semi-planing speeds. Speed is now a function of power to weight ratios. Shape of hull, lifting rails, angle of drive train force, and trim tabs will determine vessel longitudinal angle to the water and its ability to lift and increase speed.

In the past few decades the trend has been to build boats with more beam than the traditional 3-1 length to beam ratio. A wider flatter section aft of amidships will provide additional buoyancy for the weight of larger engines and payload. At all speeds the fine entry and generous dead rise forward of amidships can provide a sea kindly ride by reducing pounding in most sea conditions. The long un-ballasted keel provides a steadying effect in a seaway in addition to protecting the running gear when grounded. Some designs incorporate a ballasted keel where a section of the keel is compartmentalized and allowed to flood (water ballast) to provide additional stability with a lower center of gravity. This is especially useful in lower speed vessels that are kept offshore in heavier weather that benefit from an increased righting moment.

Downeast hull types are typically described as being either “built down” or “skeg built.” (the keel being referred to as the skeg) In the built down type, the shape of the hull forms a graceful reverse curve with considerable dead rise forward and greatly reduced moving aft. The interior of the hull amidships is full and deep allowing the engine to be placed lower in the bilge with a more horizontal drive line. With skeg built boats, the hull is shallower amidships with a flatter bottom and a vertical keel at near right angles to the hull fully aft.This hull shape is capable of higher speeds from semi-displacement to planing with an increase in power. Typically the bow is elevated slightly and a softer ride is provided. In following seas which can be the most difficult to steer in, the long keel will track well and the large rudder will provide plenty of grip on the slip stream in either type.

Other variants of the Downeast hull are found in the chine (the transition point from the top of the deadrise to hull side.) Deadrise is the angle of the hull at various points to vertical. The steeper the dead rise angle, the softer the ride. The chine in most traditional boats are softened or rounded. This make a very smooth and pleasing looking transition at the waterline. These boats typically have a slower and more comfortable roll moment or roll. Some boats have been designed with a hard chine or sharp angle where the hull side transitions to the underbody. This creates extra buoyancy and has the effect of producing less roll under way. The roll moment (roll period) is a little quicker in a hard chine boat but works very well in the Downeast design such as the Wesmac line of boats. The chine can also be used to create extra lift from amidships aft, lifting the hull slightly and allowing for greater speed with the same amount of power. Spray rails applied forward and then running aft down the slip stream, under the water line to the stern will produce the same desired effect.

Downeast boats are typically built with a single engine, gas or diesel inboard, and both have their benefits. A few Downeast boats have been fitted with twin engines but that is rare except for the modern high speed twins such as the Little Harbor. The major benefits of a single engine boat are the full length keel with center line drive and good protection of the prop and rudder in addition to the reduced expense of upkeep and efficiency with single engine operation . Cruise speeds range from displacement speed with low power to well over 20 knots cruise and higher depending on weight to power ratios. The 30′ Duffy configured to race, reached a top speed of 70 mph.

Maneuvering a Downeast boat in tight quarters and docking takes a bit of practice like any other single engine boat. With a starboard helm, the propeller is typically left hand turning in forward which will pull the stern to the right in reverse (when not making way.) This is known as prop walk.

This is a general introduction to most of the Downeast Boat designers and builders. We encourage the correction, enhancement and addition to this information on any Downeast design. Our goal is to make this an accurate source of information on Downeast lobster boats and yachts.